Measuring devices are already known for allowing the builders of injection systems and heat engines to develop injectors as well as perform adjustments and compliance verifications during production and during the installation of these systems for their final use. These measuring devices are used in conjunction with a specific test block, the role of which is primarily to drive the rotation of an injection pump and support the different elements of the injection system during tests. The measurements done with these types of devices must make it possible to precisely know both the values of injected fuel volumes and the injection times or angles.
To that end, already known from French patent FR 2795139 A or its equivalent, European patent EP 1187987 B1, in the Applicant's name, is a device making it possible to instantaneously analyze the step-by-step injection rate provided by an injection system used in a heat engine, this device being characterized essentially by the combination of two measuring chambers.
Thus, the device mentioned here includes a first chamber for measuring a constant volume chamber in which the fuel is injected, with which chamber a pressure sensor and a temperature sensor are associated that respectively measure the pressure and the temperature reigning in that chamber, as well as means making it possible to at least partially empty said first measuring chamber.
This device also includes, downstream of the first measuring chamber, a second measuring chamber in which the fuel drained from the first measuring chamber is sent, the volume of the second measuring chamber varying depending on the movement of a piston whereof the movement is measured using a movement sensor.
An electronic section controls the device assembly, said section also analyzing the information received from the different sensors.
The operating principle of this device is as follows:
When the device is ready to perform a measurement, i.e. when fuel is present in the two measuring chambers and a predetermined set pressure reigns in the first measuring chamber, an injection is done. This causes an increase of the pressure in the first measuring chamber, related to the amount of injected fuel, the characteristics of the fuel, the environmental conditions, and in particular the temperature and the initial pressure, and the volume of the chamber. At the end of the injection, the fuel contained in the first measuring chamber is partially emptied into the second measuring chamber, the pressure in the first measuring chamber thus being brought back to its initial set value and this first measuring chamber being ready then to receive a new injection. The fuel that arrives in the second measuring chamber causes the volume to increase in this second measuring chamber, by pushing the piston. The movement of the piston is measured and, knowing the diameter of the piston, a part of the electronic section calculates the exact volume of the fuel. This second measurement allows the electronic section to calibrate, at any moment and very accurately, the measurements that are done by the first measuring chamber.
In one advantageous embodiment of the device, described in the aforementioned patent documents, a rapid electrovalve controlled by a portion of the electronic section, and a discharging device, are positioned between the two measuring chambers to partially drain the first measuring chamber after an injection until this first measuring chamber returns to the pressure that reigned therein before the considered injection. In this case, the electronic section also advantageously includes a compensation function, making it possible to take a potential pressure difference into account in the first measuring chamber after two successive emptyings.
The term “fuel,” used here to qualify the fluid used in the device, in particular the fluid filling the two measuring chambers and also the injected fluid, designates an actual fuel or, preferably, a fluid that has hydraulic characteristics close to those of a fuel but with a flash point temperature much higher than that of a fuel in order to minimize the risks of fire and explosion.
Overall, the first measuring chamber, with a constant volume, makes it possible to precisely provide the “form” of the injection, while the second measuring chamber, with a variable volume, makes it possible to measure the amount of fuel injected. The processing done in the electronic section makes it possible to offset the flaws of each of the measurements by the qualities of the other.
The existing device, recalled above, is more particularly adapted to injectors that deliver a low or average amount of fuel, typically up to 100 liters per hour.
To develop high power injectors and engines, like those used to propel ships or to drive large electric generators, it is necessary to be able to perform instantaneous injection rate measurements, step by step, for more significant rates.
The production to that end of a larger measuring device, simply homothetic of the known device recalled above, faces difficulties in making the second measuring chamber, i.e. the variable-volume chamber used to perform a volume measurement via the movement of a piston.
For the first measuring chamber, which has a constant volume and is used to perform an instantaneous measurement of the pressure increase during the injection into this volume already filled with fluid, there is no technical difficulty in simply increasing the dimensions to adapt it to a higher rate. Instead of a volume typically smaller than a liter, unique to earlier embodiments, it is easy to provide a volume with a higher value, adapted to the injection pump and/or the injector that is found in the test. The value of this volume is to be determined, so as to obtain a typical increase of the pressure of several bars or tens of bars, in the first measuring chamber, during a single injection, which leads to a typical volume of several liters or tens of liters for this chamber, without these values being limiting. Thus, there would not be any drawback in principle of using still much larger volumes, if necessary, to measure very large instantaneous rates. The production of such a volume in fact remains simple, and does not pose any particular technical problems. It may potentially involve making a relatively thick, and therefore heavy, piece, due to the fact that it must resist an internal pressure that may typically go up to as much as 200 bars, but these conditions or requirements are not unusual for instrumentation of the type concerned here. Moreover, since it involves making a constant-volume chamber, without mobile parts or other delicate elements, this part will not be expensive and it will be particularly robust, regardless of its interior volume.
However, regarding the second, variable-volume measuring chamber with interior piston, difficulties arise, because this chamber must meet very strict technical imperatives, the main imperatives being:                The piston must slide perfectly, without locking or leaks, in the cylinder that delimits the measuring chamber, while the overall temperature of this chamber is generally kept relatively low, typically between 40 and 70° C., but the instantaneous temperature at the nose of the injector is high and can exceed 200° C. for modern injection systems with very high pressure, typically greater than 2000 bars.        The piston must be as light as possible in order to react quickly to the volume variations resulting from the emptying of fuel in the measuring chamber, which leads to making the piston with a hollow configuration and a very small wall thickness.        The assembly formed by the cylinder that delimits the measuring chamber, and by the piston slidingly mounted in that cylinder, must, however, be very robust to bear a large number of fuel injection cycles, therefore filling/emptying of said measuring chamber with movement of the piston, without damage.        
Pistons are usually made whereof the diameter is between 10 and 35 millimeters, and that describe a travel between 1 and 10 millimeters, which corresponds to a unitary measuring volume between about 80 and 10,000 mm3. It is common experience for the production difficulties to increase, both when one tries to make pistons with a diameter smaller than 10 mm and when one wishes to manufacture pistons with a diameter greater than 35 mm. In particular, a diameter increase of the piston, in order to adapt the device and in particular its second measuring chamber to high rates or volumes, would therefore not be a satisfactory solution.